He began working as a child actor in depression era Australia, taking his first major role in The Silence of Dean Maitland, one of Australia's first talking films.
After serving in the Second World War, Kerr moved to England to further his acting career, and during the 1940s he was regularly featured in the BBC radio series Variety Bandbox. His trademark was his catch phrase "I'm only here for 4 minutes..."
In the 1950s, he had a recurring role as an Australian lodger in the BBC radio comedy series Hancock's Half Hour. Initially sharper than Hancock's characterisation, it was developed into a more dim-witted character who became the butt of Hancock's jokes. His television appearances in Britain include a 1968 episode of Doctor Who called The Enemy of the World, with Patrick Troughton, and a long running part in the early 1960s BBC-TV soap, Compact.
Bill Kerr had much theatrical success in Britain, playing The Devil in the original West End production of Damn Yankees, directed by Bob Fosse, and Cole. In 1972 he co-starred with Anthony Newley in the long running Newley/Bricusse musical, The Good Old Bad Old Days.
He also made several films in Britain, including The Dam Busters and The Wrong Arm of the Law, before moving back to Australia. Although probably best known as a comic actor, and especially for his appearances in Hancock's Half Hour, he has since played a number of serious roles, notably in Peter Weir's films Gallipoli (1981) and The Year of Living Dangerously (1982). He also worked on the Australian stage in the 1980s, in musicals such as My Fair Lady, where he received excellent reviews as Alfred Doolittle. In 2001, he appeared in the Australian comedy Let's Get Skase.
Kerr also appeared in the 1998 television comedy series Minty.
Kerr now lives in Perth, Western Australia, with his family.